1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus and method for cleaning an ion source chamber used in ion implantation equipment.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. An ion implanter generally includes an ion source chamber which generates ions of a particular species, a series of beam line components to control the ion beam and a platen to secure the wafer that receives the ion beam. The beam line components may include a series of electrodes to extract the ions from the source chamber, a mass analyzer configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer, and a corrector magnet to provide a ribbon beam which is scanned over a wafer surface orthogonally with respect to the ion beam to implant the ions into the wafer substrate. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy.
An ion source chamber typically includes a heated filament which ionizes a feed gas introduced into the chamber to form charged ions and electrons (plasma). The heating element may be, for example, an indirectly heated cathode (IHC). Different feed gases are supplied to the ion source chamber to obtain ion beams having particular dopant characteristics. For example, the introduction of H2, BF3 and AsH3 at relatively high chamber temperatures are broken down into mono-atoms having high implant energies. High implant energies are usually associated with values greater than 20 keV. For ultra-shallow ion implantation, heavier charged molecules such as decaborane, carborane, etc. are introduced into the source chamber at a lower chamber temperature which preserves the molecular structure of the ionized molecules having lower implant energies. Low implant energies typically have values below 20 keV. Certain ion chambers may be configured to provide either mono-atoms or heavy molecular species by using different feed gases and extraction components. Alternatively, certain ion chambers may be configured to supply both high implant energy mono-atoms as well as low energy implant molecules for implantation into a semiconductor substrate through the use of various extraction components. However, when a particular feed gas is supplied to the source chamber to produce a desired ion species, additional unwanted species, either ions or neutrals, may also be produced. These unwanted species typically have a very low vapor pressure and may condense and adhere to the interior surfaces of the source chamber. For example, when phosphine (PH3) is fed into the source chamber, phosphorous (P) deposits may form on the chamber walls. When heavy molecules such as decaborane and carborane are fed into the source chamber, unwanted deposits on the source chamber walls and electrodes is more prevalent. These solid deposits may change the electrical characteristics (voltage instability) of the chamber walls and possibly interfere with the chamber aperture from which the ions are extracted, thereby causing unstable source operation and non-uniform beam extraction. In addition, dopant cross contamination may occur when the use of one type of feed gas in a cycle contaminates the source chamber for subsequent cycles using a different feed gas. Source chambers that generate mono-atoms may run for a week or more before cleaning is necessary. Conversely, source chambers that generate heavier molecular species may require cleaning after just a few hours of operation. Thus, source chambers that operate in dual mode (mono-atom and molecular species) may require more frequent cleaning.
One method used to clean the ion source chamber includes the introduction of a cleaning gas such as, for example nitrogen triflouride (NF3) or sulfur hexaflouride (SF6) which etches away the unwanted deposited material via plasma-enhanced chemical reaction and exits the source chamber as a gas. Introduction of theses cleaning gases is performed in situ and may be introduced simultaneously with the dopant species or as a separate cleaning plasma during equipment down time and/or between specie changes. In order to provide stable source operation and avoid cross-contamination, as much as 4-5 cleaning cycles may be required to remove certain unwanted deposits. Thus, it is desirable to enhance chamber cleaning to increase efficiency and unnecessary equipment downtime. The cleaning process can be tuned by altering the temperature of the reaction in the chamber and the flow rate of the feed gas. In addition, increasing the pressure inside the chamber during the cleaning cycle is another method to enhance the cleaning process. Plasma reaction at higher pressures normally promotes the production of active neutral species (e.g. F*) which is an effective deposit etchant. However, it is difficult to increase the pressure in the source chamber to sufficiently high levels because of the loss of gas, for example, through the source chamber aperture. Moreover, the aperture can not be reduced to increase chamber pressure during cleaning because the extraction current is proportional to the aperture area which is particularly important for molecular beam extraction processing.